Bleaching a paper web with peroxide



FIPB3D seen 2,893,819 BLEACHIN G A PAPER WEB WITH PEROXIDE Alfred T. Hawkinson, Niagara Falls, N.Y., assiguor to E. I. du Pont de Nemours and Company, Wilmington, 'Del., a corporation of Delaware No Drawing. Application August 8, 1955 Serial No. 527,129 2 Claims. (Cl. 8-111) This invention relates to a means for drying a paper web which has been treated with peroxide and more particularly to a paper bleaching process wherein a paper web is treated with peroxide and then heated to accelerate the bleaching action of the peroxide and to dry the paper.

Processes have been proposed for bleaching paper in a continuous manner by applying a peroxide bleaching agent to a paper web and then heating the web to accelerate the bleaching reaction and to dry the web to any desired extent. In such processes the paper web may be wet or dry at the time the peroxide bleaching agent is applied and the peroxide bleaching agent maybe applied in a variety of ways. In order to bleach paper with a peroxide or equivalent peroxygen compound, the bleaching agent must be contacted with the pulp in the form of an aqueous alkaline solution. Thus, if desired an aqueous alkaline peroxide solution may be applied to the paper web and the latter then heated and dried. In an alternative method the web may first be treated with an aqueous alkaline solution and thereafter treated with a peroxide, for example a hydrogen peroxide solution. In a preferred method, which is disclosed in the pending US. patent applications Young, S.N. 358,503, filed May 29, 1953, now US. Patent 2,777,749, and Hawkinson et al., S.N. 497,432, filed March 28, 1955 now US. Patent 2,859,087, the two-stage application is carried out by first applying to a paper Web an aqueous alkaline solution and then treating the moist web with hydrogen peroxide vapor. The so-treated web is then heated to accelerate the bleaching action and to dry the web to any desired extent.

In the heating and drying of the peroxide treated web in practicing the above-mentioned processes, conventional drying equipment such as that utilized in the Fourdrinier paper making machine may be utilized. This drying equipment consists of a series of rotating heated drier rolls over which the paper is passed. To obtain adequate contact of the paper with each heated roll, an endless, vapor permeable belt or drier felt, usually made of cotton fabric, is arranged by means of suitable idler rolls so as to embrace, and move with a substantial portion of the periphery of each heated roll, generally a single felt being utilized for a plurality of drying rolls. By conventional means, each felt is held taut, so that it is held tightly against the drier roll. The paper web then is fed between the drier felt and the drier roll whereby the felt holds the paper tightly against the roll, thereby accomplishing adequate heat transfer from the roll to the paper. The water vapor generated by the heating of the paper escapes through the porous construction of the cotton fabric, drier felt.

The term felt, as generally used in the paper industry and as used herein refers to those endless textile webs in paper machines that support the paper web and hold the paper against drier rolls or press rolls. The felts fall into two classes, functionally: (a) wet felts and (b) drier felts. The wet felts support the wet paper web leaving the paper forming screen and carry it through press and suction rolls where much of the water is mechanically removed. Wet felts generally are woven of wool and are relatively thick, soft, spongy and resilient. Conventional drier felts, whose function is described 2,893,819 Patented July 7, 1959 above, are of heavy woven cotton construction with a relatively smooth, hard surface and a minimum of stretch and resiliency.

In the conventional paper making operations, the cotton drier felts have adequate durability for their purpose. However, in peroxide bleaching operations wherein a wet paper web treated with peroxide and alkali is heated and dried by means of paper machine drier rolls, the cotton drier felt is seriously damaged by contact with the peroxide on the paper. Cotton felts so employed become damaged to such extent as to require replacement within 20 to 30 hours of operation, whereas under ordinary conditions of paper making without application of bleaching chemicals on the paper, the same felt has a normal life of 3 to 12 months.

I have found that drier felts made of polyester fiber and having about the same thickness and construction as conventional cotton drier felts can be utilized in the above-mentioned bleaching processes for long periods of time with no apparent damage. The results so far obtained indicate that the perioxide and alkali applied to the paper has substantially no damaging effect on the polyester felts and that they can be used in such bleaching operations for periods of time at least as long as the life of cotton felts in ordinary paper-making operations. Comparative experiments have also indicated that felts made of other synthetic fibers, like cotton, are also damaged by the peroxide and alkali in the bleaching process, under conditions which cause no apparent damage to the polyester fiber felts.

Example 1 To test the elfect of alkali and peroxide on paper machine drier felts made of cotton, laboratory tests were made, simulating the conditions existing on paper machine drier rolls. A hollow steel cylinder was heated by introducing steam therein at 10 lbs/sq. in. gauge pressure (ca 230 F.). to a consistency of about 25% by weight of water to 25 of dry fiber) in an aqueous solution containing 0.5% of sodium pyrophosphate and 0.5% by weight of hydrogen peroxide. Each sheet was dried by placing it on the hot cylinder and holding it thereon for 30 seconds by the pressure of piece of new (unused) cotton dryer felt held taut around the cylinder. The felt was then dried on the cylinder for 30 seconds. This cycle was repeated, until 10 pieces of paper had been thus dried, using the same piece of felt.

The above procedure was then repeated with paper soaked in pure water, using another section of new dryer felt, except that (a) two pieces of paper were dried simultaneously, (b) the drying time was 1.5 minutes and (c) the procedure was repeated for a period of about 8 hours.

A small section then was cut from the center of the two pieces of dryer felt and these samples were subjected to the Cuen fluidity test, to measure degradation suffered by the cotton. Also the Cuen fluidity test was applied to samples taken from a cotton dryer felt which had worn out in service on a paper machine and a new, unused cotton dryer felt. The following Cuen fluidity results were obtained:

Cotton Felt Tested Treatment Fluldlty (Rhes) New Felt Used Felt New Felt Nrm a do UsedtS hrs. to dry paper soaked in pure wa er. Used to dry paper soaked in peroxide 30 solution (10 cycles).

Paper newsprint sheets were soaked Example 2 The procedure of Example 1 was repeated, except that in one test, a piece of thin polyacrylonitrile (Orlon) fiber fabric was used in place of cotton dryer felt and in another test a thin polyester (Dacron) fiber fabric was used. In each of these tests, 200 pieces of paper were dried after wetting with the alkaline peroxide solution and the fabric pieces were dried after drying 4 pieces of paper. The polyester fiber fabric was made from polyethylene terephthalate staple.

Separate samples of the polyacrylonitrile and polyester fabric were scoured for 30 minutes at 100 C. with an aqueous soap solution containing 1% of soap and 1% of Duponol ME (a commercial wetting agent manufactured by E. I. du Pont de Nemours and 00.), these percentages based on the fabric weight.

By a standard procedure, the tensile strengths of these four pieces of fabric then were determined, and also the tensile strengths of untreated pieces of the same fabrics. The following results were obtained:

Tensile Strength 1 (lbs.)

Loss in Tensile Strength, percent Fabric Treatment None Scoured Used to dry paper wet with peroxide solution.

None D scour Used to dry paper wet with peroxide solution.

1 Average of 5 tensile strength tests on each sample.

The polyacrylonitrile and polyester fabrics used were not of the same weight and construction, which accounts for the comparative tensile strength of the untreated samples.

Example 3 drogen peroxide vapor was applied to the paper between the second press rolls and the first dryer roll. The cot- ,ton dryer felts were badly damaged in these operations and had to be replaced after 20 to 30 hours of operation.

The machine was then equipped with Orlon polyacrylonitrile fiber felts having the same thickness and construction as the cotton felts. These failed after about 100 hours of operation under the above-described conditions.

The Orlon acrylic fiber felts then were replaced with Dacron polyethylene terephthalate fiber felts of the same construction and thickness and operation was resumed as described above. The Dacron fiber felts have shown no evidence of deterioration after more than 200 hours of operation in the above-described conditions.

The polyester fibers used to make the drier felts employed in practicing my invention are synthetic fibers made from the polymeric interaction of a glycol and a dibasic acid, as described in U.S. Patents 2,465,319 and 4 U.S. 2,465,150, for example polyethylene terephthalate fibers made from ethylene glycol and terephthalic acid.

A variety of such fiber forming polyesters are known and the production of such fibers is not a part of my invention.

While the present invention has been described particularly with respect to the drying equipment utilized in a conventional Fourdrinier paper machine, it must be understood that the invention is not so limited but is applicable to any paper bleaching operation in which a paper sheet treated with alkaline peroxide is heated by contact with a heated surface, wherein the paper is held in close contact with said heated surface by the pressure of a taut fabric constructed of polyester fiber. For example, the paper may be heated by passage over the periphery of a single large drying roll or similarly over a plurality of such drying rolls but in each case the paper is held against the periphery of the drying roll by a taut web constructed of polyester fibers. The construction of the polyester felts may be similar to that of conventional woven fabric cotton drier felts or other suitable construction. Either spun yarn or continuous filament polyester yarn may be used to make the felts.

The polyester fiber drier felts must have sufficient vapor permeability to permit escape through the felt, of water vapor evaporated from the paper. This is because the polyester fibers, unlike cotton fibers, are hydrophobic and have little tendency to absorb water. Whereas the cotton felts have to be passed over one or more extra drier rolls to evaporate out absorbed water, such drying of the polyester fiber felts may be dispensed with.

The bleaching operations described above have utility over conventional bleaching operations in which the paper pulp is first bleached and then made into paper, because of the limited equipment required to bleach the paper web on conventional paper-making machinery or drying machinery, as compared with the equipment required to bleach pulp in unfabricated form, and because of the high speed at which the bleaching operation can be carried out. Those advantages tend to greatly decrease the cost of producing a bleached paper. However, such economic advantages are to a large part neutralized by the rapid destruction of conventional cotton drier felts in carrying out the bleaching process. By use of the polyester fiber felts, the full economic advantage of the new bleaching process can be achieved.

I claim:

1. A bleaching process comprising separately applying to a moving paper web an aqueous alkaline liquid and hydrogen peroxide and drying the so-treated web by bringing it into contact with at least one rotating heated roll, while pressing the paper web against the heated roll surface by means of a taut, vapor permeable web constructed of polyethylene terephthalate fiber.

2. The process of claim 1 in which the roll surface, paper web and polyethylene terephthalate fiber web move at substantially the same speed.

References Cited in the file of this patent UNITED STATES PATENTS 2,037,119 Comey Apr. 14, 1936 2,426,142 Campbell Aug. 19, 1947 2,554,034 Koester May 22, 1951 2,612,190 Hall Sept. 30, 1952 OTHER REFERENCES Dacron Technical Manual, Du Pont de Nemours, Wilmington, DeL, esp. pp. 12.0l, 23.01, 23,0 2, July 21, 1952. 

1. A BLEACHING PROCESS COMPRISING SEPARETLY APPLYING TO A MOVING PAPER WEB AN AQUEOUS ALKALINE LIQUID AND HYDROGEN PEROXIDE AND DRYING THE SO-TREATED WEB BY BRINGING IT INTO CONTACT WITH AT LEAST ONE ROTATING HEATED ROLL, WHILE PRESSING THE PAPER WEB AGAINST THE HEATED ROLL SURFACE BY MEANS OF A TUART, VAPOR PERMEABLE WEB CONSTRUCTED OF POLYETHYLENE TEREPHTHALATE FIBER. 